John Kerry attends nuclear talks in Switzerland
John Kerry attends nuclear talks in SwitzerlandReuters

World powers seeking to pin down a nuclear deal with Iran will hold their first full meeting with the Iranian delegation during talks in Switzerland later Sunday, US officials said.

US Secretary of State John Kerry, German Foreign Minister Frank-Walter Steinmeier, France's Laurent Fabius as well as China's Wang Yi and EU High Representative Federica Mogherini will meet their Iranian counterpart Mohammad Javad Zarif and political directors from Russia and Britain, an American official said.  

The news comes shortly after US Secretary of State John Kerry cancelled plans to leave for an event in Boston on Monday in order to keep negotiating, the State Department said.

His French and German counterparts, Laurent Fabius and Frank-Walter Steinmeier, both due in Kazakhstan on Monday, have followed suit, diplomats said.

Chinese Foreign Minister Wang Yi arrived in Lausanne on Sunday morning. Russian and British top diplomats Sergei Lavrov and Philip Hammond were expected later, completing the line-up of foreign ministers from the six powers.  

Kerry met again early Sunday with Iranian Foreign Minister Mohammad Javad Zarif, the latest in a flurry of closed-door discussions at a luxury hotel in the Swiss town.  

Asked afterwards if he was going to get a deal, Kerry said: "I don't know."

"It's going all right. We're working," he added.  

In Israel, meanwhile, Prime Minister Binyamin Netanyahu opened Sunday's cabinet meeting with a dire warning about the impending deal, saying it appeared the proposal up for discussion is even worse than Israel had previously thought.

"The dangerous accord which is being negotiated in Lausanne (Switzerland) confirms our concerns and even worse," Netanyahu said inremarks at a meeting of his cabinet broadcast on public radio.

"Even as meetings proceed on this dangerous agreement, Iran's proxies in Yemen are overrunning large sections of that country and are attempting to seize control of the strategic Bab-el-Mandeb straits which would affect the naval balance and the global oil supply," he said of Tehran's aggressive actions in the region.

"After the Beirut-Damascus-Baghdad axis, Iran is carrying out a pincers movement in the south as well in order to take over and conquer the entire Middle East. The Iran-Lausanne-Yemen axis is very dangerous for humanity and needs to be stopped." 

Israel, along with neighboring Sunni Arab states, is concerned that a deal that six powers are trying to agree the contours of by midnight on March 31 will fail to stop Iran from getting the bomb, and amount to "appeasement" of the Islamic Republic.

P5+1 negotiators on the other hand have expressed guarded optimism that after 18 months of tortuous negotiations and two missed deadlines that a breakthrough might be in sight for a deal ending 12 years of tensions.

"If we manage to resolve all the remaining issues today or in the next two to three days, then we can begin to draw up a text. But for the moment we are still in discussions," a source close to the Iranian delegation said Sunday.

Steinmeier said Saturday the talks were in the "endgame" but added that "the final metres are the most difficult but also the decisive ones".  

The aim is to agree broad outlines for an accord by Tuesday's midnight deadline, and then flesh out a series of complex annexes containing all the technical details by June 30.

The mooted deal would see Iran scale down its nuclear program and allow unprecedented inspections of its remaining activities.

The hope is to prolong the theoretical "breakout" time that Iran would need to produce enough fissile material to build a nuclear bomb to at least a year from the current estimate of several months.

This would require a combination of slashing the number of machines producing nuclear material, converting the capacities of existing nuclear plants such as the underground Fordo facility, exporting its stocks of enriched uranium and limiting the development of newer, faster equipment.  

But Iran is insisting that in exchange global powers must lift sanctions that have choked its economy by strangling its oil exports and banks.

Critics of the negotiations have claimed that the deal under discussion would extract practically no concessions from Iran - requiring it only to pause certain elements of its existing program for a limited period - in return for western capitulation on sanctions.

The issue of UN sanctions is proving particularly thorny, diplomats said, with global powers insisting the sanctions should be eased only gradually to ensure that they can be "snapped" back into place if Iran violates the deal.

"The brinksmanship in these negotiations will no doubt continue until the 11th hour," said Ali Vaez, an expert at the International Crisis Group.

Kerry is under pressure to return from Lausanne with something concrete to head off a push by Republican lawmakers to introduce yet more sanctions, potentially torpedoing the whole negotiating process.

Russia has also warned that US-supported airstrikes by Iran's foe Saudi Arabia on Iran-backed rebels in Yemen were "having an impact".

"We hope that the situation in Yemen will not bring about a change in the position of certain participants," said Russia's chief negotiator, Sergei Ryabkov, quoted by Ria Novosti news agency.

AFP contributed to this report.